The present invention relates to a method for producing ceramic powders.
Examples of general methods of this type are electric arc melting processes such as described in an article by H. Thomas, Ceramic Technology International, 1992, pp. 65-68. However, in response to the heat treatment, in particular, in response to the melting of ceramic powders, gas cavities and gases from the ambient environment (in particular, nitrogen from the ambient air) can be embedded in the crystal lattice of the melting stock, obtained as intermediate product, or of the ceramic body. After this intermediate product is further processed into a ceramic powder, for example, through grinding, it is possible that in the subsequent sintering of the ceramic powder a swelling of the ceramic structure can arise as a result of gas expulsion (see M. Merch, J. Am Ceram. Soc, 79, (1996), p 2641-44 and M. Merch et al., J Mat Sci Letters, 15, (1996), p. 2127-29). This problem arises, in particular, in the manufacture of planar sensor elements on a base of ZrO2 stabilized by Y2O3. The ceramic powders used for this purpose are often produced in an electric arc melting process, followed by grinding and particle size fractionation, and are then sintered, after shaping. Ceramic powders are particularly suitable for planar sensor elements due to their favorable properties with respect to film casting and on the basis of their favorable sintering activity.
To avoid undesirable gas cavities or the embedding of gas atoms in the ceramic base material during melting, the melting process has previously been partially carried out in a protective atmosphere, so that, for example, a displacement of the ambient air from the base powders or from the ceramic raw ore can be achieved before and during the actual melting process. In this context, however, areas still regularly arise that have undesirably high concentrations of residual gas or gas cavities (so-called xe2x80x9cair pocketsxe2x80x9d) in powder agglomerates in the raw ore.
An alternative, familiar process, also having.t.he goal.of nitrogen expulsion, is, for example, the calcining of a solidified or broken up melting stock made of yttrium-stabilized zirconium dioxide at roughly 1600xc2x0 C. in an oxidizing atmosphere, before the subsequent grinding or classifying process. This method of re-calcining ceramics in order to increase the sintered density is described by Esper and Friese (DKG conference 1972, proceedings XXIX, pp. 1-13) in the example of pure clay, which contains xcex2-pure clay and which in turn decomposes into Na2O and Al2O3, in the process being able to expand the ceramic structure due to the vapor pressure that arises. Some disadvantages of these methods, however, are found in the great technical expense of a further calcining step and in the often insufficient subsequent displacement of the residual gases from the ceramic powder or from a corresponding intermediate product.
An object of the present invention is to produce ceramic powders that result from a heat treatment of the base powders, such that existing gas cavities or gas components are to a high degree displaced from the ceramic base powder or the ceramic slurry before or during the production process. In addition, as according to the present invention adjusts in a controlled manner certain partial pressures of one or more desirable gases in the resulting ceramic powder.
In contrast to the related art, the method according to the present invention has an advantage in that it is significantly more rational and effective with respect to the completeness of the gas displacement or of the partial pressure adjustment and to the technical process control. Thus, using solid or liquid additives, which, for example, experience a significant increase in volume in response to evaporation or sublimation or thermal decomposition, a gas is largely or completely displaced from the ceramic base powder or powders or from the slurry as a base material, using a relatively small quantity of additives. Furthermore, using the additives, gases can be introduced into the ceramic powder in a controlled manner, gases that were not contained in the ceramic base materials, so that the method according to the present invention makes possible a broad palette of possibilities for modifying the resulting ceramic powder in its properties, and specifically in its sintering characteristics. This manner of proceeding makes possible, for example, a controlled adjustment of an oxygen partial pressure or the doping of a ceramic or of a ceramic powder having oxygen voids, which has great technical importance, in particular, in the area of ceramic sensor elements on the basis of oxygen-ion-conductive, yttrium-stabilized ZrO2. Furthermore, through the addition of corresponding additives, the sintering activity of the resulting ceramic powder can be improved in its further processing.
A gas can be substantially displaced particularly advantageously, for example, from powder agglomerates in the raw ore or from gas cavities in a ceramic slurry, or a partial pressure can be adjusted, if the additives have been added to the base powder as grinding aids already before the actual transformation and/or are introduced, in the form of a binder system, into the ceramic slurry or the ceramic powder. In one embodiment according to the present invention, using this treatment, it is possible to achieve a pelletizing or granulizing of the base materials before the actual transformation of the additives in the course of a heat treatment of the base powders or slurry. As a result of preparatory steps of this type, the additives can be distributed very uniformly in powder agglomerates or in ceramic slurries, and the powder can be diffused sufficiently loosely, which, for example, paves the way for a rapid and adequate degasification of a powder raw ore. By adding a plurality of different additives, the possibility arises that gases such as air or nitrogen, during the transformation of the additives, can be displaced by a heat treatment of the base materials over a very broad temperature range, thus avoiding an explosion-like gas expansion. Such combinations of additives can be advantageously realized, for example, using solutions, emulsions, or suspensions, such as are known in ceramic processing technology.
In the event that use is made of a mixture of a Y2O3 base powder and a ZrO2 base powder, it is possible to obtain, for example, using the method according to the present invention, a nitrogen-poor lattice structure and consequently a high sintered density of the yttrium-stabilized ZrO2 ceramic produced, without a disadvantageous swelling up of the ceramic structure in response to sintering. One embodiment of the method that is technically particularly simple and advantageous results if the fine-particle-sized ceramic ZrO2 and Y2O3 base powders have added to them the corresponding hydroxides before or during the heat treatment or, in the concrete case, before or during an electric arc melting, the hydroxides being then transformed into the corresponding oxides releasing water vapor, the resulting water vapor displacing the air or oxygen from the powder raw ore and, optionally, at high temperatures further decomposing into hydrogen and oxygen.